Oil recovery process using an emulsion modifier-containing dilute aqueous surfactant system

ABSTRACT

A process for displacing oil within a subterranean reservoir by injecting a surfactant system that contains an emulsion modifier that either prevents the formation of an emulsion or reduces the bypassing of oil due to the formation of an emulsion.

O C :1 D H SUBST'l 111m FOR MISSING COPY Hill et a]. 1 Jan. 23, 19731541 OIL RECOVERY PROCESS USING AN 3,348,611 /1967 Reisberg ..l66/274 xEMULSION N NING 3,477.51 1 11/1969 Jones et a1 ..166l274 3,493,0482/1970 Jones ..166/252 DILUTE AQUEOUS SURFACTANT 3,506,070 4/1970 Jones..166/274 X SYSTEM 3,508,612 4/1970 Reisberg et a1 ..166/Z74 D R3,536,136 10/1970 Jones ..166/252 [75] Inventors {il st g f 05s lgpen3,589,444 6/1971 Johnson et a1... ..166/274 3,605,891 9/1971 Ayers, Jr...-..-......166/273 [73] Assignee; Shell Oil Company, Houston, Tex,3,637,017 1/1972 Gale et a1. ..166/274 3,638,728 2/1972 Hill ..166/273[22] Filed: Oct. 26, 1971 [2 App. No: 192 129 Primary Examiner-StephenJ. Novosad Attorney-Harold L. Denkler et a1.

[52] 11.5. C1 ..l66/252, 166/273 57] ABSTRACT [51] Int. Cl. ..E2lb 43/22[58] Field of Search ..l66/252 250 273-275 A Pmess dsplacmg a wmemneareservoir by injecting a surfactant system that contains [56] ReferencesCited an emulsion modifier that either prevents the forma- A1 tion of anemulsion or reduces the bypassing of oil due UNITED STATES PATENTS tothe formation of an emulsion.

3,330,344 7/1967 Reisberg ..l66/274 Claims, 5 Drawing Figures OFRES/DUAL e g B CORE 776 Z CORE 775 g l I EMULSION 08- b SERVED IN 8PRODUCED FLU/D5 k l 0 1/ i 1 1 1 1 CUMULATIVE INJECTION OF CHEM/CALFLOOD (FORE VOLUME) PAIENTEDJAH 23 ms =snm1ur4 F I G. I

g l g 70- t [L 60 J 40- CORE 776 z coRE 775 u EMULSION 08- g l SERVED /Ng PRODUCED FLUIDS Q 0 1 J l l o I n 1 1 0 0.4 0.0 1.2 7.6 2.0 CUMULATIVEINJECTION OF CHEMICAL FLOOD (PORE VOLUME) CORE 775 coRE 775 k a 0.6 2 BS Lu 0.5- P u PE 3 04 5 m 0.3- /o Q E g g 02- 7 D Q o 8 no: a, r W o:L9; 0 l 1 1 l V CUMULATIVE INJECTION OF CHEMICAL FLOOD (PURE VOLUME)PAIENTEUJAH 23 I975 SHEET 2 or 4 CHLORIDE ION IN CORE 6 5 7 7 7 7 v E Em R L C 0 C w W L A, M E m w w am R w mm NA R I 6 EW A U K N L m I M E m2 I. 0 1. 0 0 0 0 QOQE Qquimlu n6 kmqkm HQ mtou 25 QmuDQOQnG 29 mQESIu80395 (PORE VOLUME) FIG.3

j Pmmmmz I975 sum 3 or 4 PARENT CHEM/CAL SLUG I CONTAINS N0 IBA O PARENTCHEM/CAL SLUG CONTAINS A IBA 03m uimru 6 \CGSWE ZQGJDEM b6 \rtmdumi O/LCONTENT, FRACTION OF TOTAL VOLUME FIG.4

OIL RECOVERY PROCESS USING AN EMULSION MODlFlER-CONTAINING DILUTEAQUEOUSSURFACTANT SYSTEM BACKGROUND OF THE INVENTION This invention relates todisplacing oil within a subterranean reservoir by injecting an aqueoussurfactant system into the reservoir.

The following terms are used with the meanings indicated. A surfactantsystem is a solution and/or dispersion of surface active materials inliquid. An aqueous surfactant system, commonly referred to as a chemicalslug is one having an aqueous liquid continuous-phase and a proportionof aqueous liquid components (solvent and/or solute) that substantiallyequals or exceeds the proportion of oil-phase components (solvent and/orsolute)..An active surfactant system is one having an interfacialtension between it and an oil-containing reservoir fluid that is lessthan about 0.01 dyne per centimeter. The oil displacement efficiency ofa surfactant system is the extent to which it effects a completedisplacement of oil-containing reservoir fluid within the part of areservoir that is permeated by the surfactant system. The sweepefficiency, e.g., in a chemical flood oil recovery process, is theextent to which the entire reservoir (within the flood pattern) iscontacted by the chemical system.

in a chemical flood oil-displacement process, chemicals inclusive of anaqueous surfactant system are injected into a subterranean reservoir todisplace oil-containing reservoir fluid within the reservoir. ln a welltreatment oil-displacing process, a relatively small chemical slug isinjected to form a ring (which becomes thinner as it is displaced awayfrom the injection well) that soon breaks up and becomes dispersedwithin the reservoir, usually when the ring has been displaced by fromabout 5 to l0 feet from the injection well. in an oil-recovery process,a relatively large chemical slug is injected to form a ring that remainssubstantially intact as it is displaced from an injection location to aproduction location, usually involving distances of from about 50 toseveral hundred feet.

The value of oil recovered from a reservoir is diminished by the cost ofchemicals injected into and lost within the reservoir. The cost (perpound) of materials used in chemical flooding processes usually decreasein the order of: thickening agents, surface active materials, oilsolvents, water-soluble salts, and water. For equal volumes ofsurfactant systems of equal activities and mobilities, the cost of anactive aqueous surfactant system is generally less than that of anoilphase-continuous surfactant system. The latter type of system iscommonly called a soluble oil, a microemulsion, or an oil-externalmicellar dispersion.

in certain reservoirs containing relatively viscous oils, the propertiesof the oil-containing reservoir fluid and the reservoir rocks are suchthat the relatively dilute and low cost aqueous surfactant systems tendto SUMMARY OF THE INVENTION In accordance with the present invention,the need for using a relatively expensive, oil-phase-extemal, surfactantsystem to effect oil displacement in a reservoir in which an activeaqueous surfactant system tends to form a relatively viscous (lowmobility) emulsion, is avoided by a combination of testing, formulatingand injecting steps. Tests are made of the oil displacement behavior ofan active aqueous surfactant system, (preferably one containing lessthan about 8 percent oil-phase material components such as hydrocarbons,surfactants, thickeners, etc.) having a mobility that is at leastsubstantially as low as that of the oil-containing reservoir fluid. Thetests are preferably made in a model or core of the reservoir at thereservoir temperature. Where such a surfactant system shows a tendencyto form a low mobility emulsion, it is modified by adding an emulsionmodifier in an amount suff cient to avoid the deleterious effects ofemulsion formation by (l) avoiding the emulsification, (2) increasingthe coalescence rate of the emulsion to a value such that completeseparation of the emulsion into its respective oleic and aqueous phasesoccurs within a few hours and/or (3) reducing the emulsion viscosity toa level such that its mobility in the reservoir will be more than themobility of the chemical slug. The emulsion modifier-containing aqueoussurfactant system is injected into the reservoir to displace oil withinthe reservoir.

An emulsion that may be formed by injecting a dilute, active, aqueoussurfactant system (or chemical slug) into a reservoir may comprise (1)oil and oil soluble chemical slug components dispersed in a continuousaqueous phase containing water soluble components of the chemical slug,(2) an aqueous phase containing water soluble components of thesurfactant slug contained in a continuous reservoir oil phase containingthe oil soluble components of the surfactant slug or (3) more complexemulsions such as an aqueous phase predominately derived from thesurfactant slug and containing a part of the water soluble componentsfrom the surfactant slug dispersed in the reservoir oil containing theoil soluble components from the surfactant slug, this entire emulsionbeing in turn dispersed in an aqueous phase predominately derived fromreservoir water but containing a part of the water soluble componentsfrom the surfactant slug. When anemulsion is formed by interactionbetween chemical slug and reservoir oil and this emulsion is less mobile(in general more viscous) than the chemical slug, said emulsion will beby-passed or fingered through by the remaining slug. Since the portionof the slug which fingers through such an emulsion encountersunemulsifled crude oil immediately upon breaking through the emulsionzone, additional emulsion isformed. This new emulsion is in turnbypassed by chemical slug with the continuing formation of emulsion. Inthis fashion, all of the chemical slug can become a component part of aviscous emulsion. The degree to which the chemical slug fingers throughor bypasses the emulsion and thus generates new emulsion is determinedby (l) the stability of the emulsion with respect to time and (2) themobility of the chemical slug relative to the mobility of the emulsion.If the ratio of chemical slug mobility to emulsion mobility is one orless, the emulsion will not be bypassed but will be pushed ahead of theslug without damage to the oil recovery process. As the ratio ofmobilities increases above I, more and more tendency for bypassingoccurs. Addition of increased amounts of mobility control agent to thechemical slug does not, in general, alleviate the harmful effects ofemulsions since the emulsion viscosity itself may be increased by theadded' thickener. in the extreme but not unusual case of a viscous andvery stable emulsion, the entire slug may become emulsified withreservoir fluids. When this occurs, the thickened slug following thechemical slug must be less mobile than the emulsion or it will bypasstheemulsion resulting in failure of the oil recovery process and loss ofthe chemical slug. For the less mobile possible emulsions, the amount ofpolymer or thickener required to provide a thickened slug with asufficiently low mobility to displace the emulsion may easily be sogreat as to seriously impair the economics of the oil recovery process.

DESCRIPTION QF THE DRAWINGS FIGS. 1 to 5 are each a plot of thevariations of one designated quantity with increases in anotherdesignated quantity.

DESCRIPTION OF THE INVENTION The introduction into a surfactant systemof an agent hours, the extent of bypassing will be negligible sincefrontal advance rates in most reservoirs is of the order of 1 foot perday. Alternatively, it may be found that one of the tested emulsionmodifiers results in a relatively stable emulsion with greatly reducedviscosity. if this reduced viscosity is low enough to be equal to orless than the slug viscosity, it will, in general, by displacedefficiently by the slug. It may be found that an emulsion modifier willserve to both reduce viscosity and increase coalesence rate. In thiscase, the combined beneficial effects may be fully realized economicallybe selecting a concentration of the modifier just barely high enough togive the desired result of efficient oil displacement from thereservoir. For example, it may be found that a given emulsion modifierat an economically low concentration will cause otherwise highly stableemulsions to coalesce in IS to 36 hours and that while these emulsionsare in existence, their viscosity is only 1.5 to 2 times the viscosityof the slug. Such properties should allow for efficient oildisplacement.

Included among the many materials useful as emulsion modifiers in thepresent invention are semipolar organic compounds, such as amines,diamines, polyethoxylated amines, amides, sulfonamides of water solublepetroleum sulfonic acids, water-soluble oil-insoluble petroleumsulfonates, ketones, alcohols, and the like. In general, semipolarorganic compounds which are useful in breaking oil field emulsions areuseful in the practice of the present invention. Preferred materialsinclude semipolar compounds, such as alcohols, polyethoxylated amines,the water-soluble, oilrelatively high oil-phase-material-content systemsare generally free of a tendency to form a low mobility emulsion withinan oil reservoir, their use is relatively expensive, and the avoidanceof a need for their use is a primary object of the present invention. Inaddition, in

' which avoids emulsification entirely is a desirable goal but mayrequire excessive amounts of costly additives,

an oil recovery operation, the present use of a relatively low cost, lowoil-phase-material-content surfactant system that contains an emulsionmodifier which avoids emulsification or increases coalesence rates ofemulsions causes most of the displaced oil to be recoverable in the formof a relatively water-free and non-emulsified liquid phase.

The components of a chemical slug used in the present invention cancontain substantially any mixture of surfactants, electrolytes, and/orcosurfactants, cosolvents, semi-polar materials and thickeners that isadapted to form an aqueous chemical slug having an interfacial tensionof less than about 0.01 dyne per centimeter against the fluids within anoil containing reservoir. Such compounds can be utilized in combinationof components such as those previously proposed for oil-displacingchemical slugs. Preferred surface active and electrolytic components aredescribed in patents such as the .l. Reisberg U.S. Pat. Nos. 3,330,344and 3,348,6ll. Suitable semi-polar organic materials are disclosed inthe S. C. Jones U.S. Pat. Nos. 3,506,070 and 3,506,071.

- The components of a thickened slug and drive fluid that may beinjected behind the chemical slug can also comprise thickening agentsand aqueous solutions containing combinations of compounds such as thosepreviously proposed for similar uses. Suitable thickeners and drivefluids are described in the patents mentioned above.

Particularly suitable surfactants comprise mixtures of alkali metalsalts of petroleum sulfonates (such as alkylaryl sulfonates, alkylatedbenzene sulfonates, and the like) and sulfated polyoxyalkylated alcoholsurfactants. Such petroleum sulfonate surfactants are commerciallyavailable for example, as Petroleum Sulfonates, from Bray ChemicalCompany, BRYTON Sulfonatcs, from- Bryton Chemical Company, PETRONATESand PYRONATES from Sonneborn Division of Witco Chemical Company, PROMORSulfonates, from Mobil Oil Company, and the like. Surfactant sulfatesofethoxylated primary alcohols are sold as NEODOLS by Shell ChemicalCompany. Other surfactant sulfates of ethoxylated alcohols are availableas TERGlTOLS from Union Carbide, and the like. Particularly suitablemixtures of sulfonate and sulfate surfactants are described in the J.Reisberg, G. Smith, and .l. B. Lawson U.S. Pat. No. 3,508,612.

Suitable water soluble thickeners for use in the chemical slug and thethickened slug comprise watersoluble polymeric materials, such ascarboxymethyl cellulose, polyethylene oxide, the high molecular weightsalts of polymers containing amide and carboxylate groups that areproduced by polymerizing acrylamide (or its homologs, such asmethylacrylamide) and partially hydrolyzing the amide groups and thelike. Particularly suitable thickeners comprise high molecular weightpolyelectrolyte polymers such as the partially hydrolyzedpolyacrylamides. Such preferred thickeners are available undertradenames such as PUSHER and SEPARAN from Dow Chemical Company. Anadditional suitable class of polymers are the biopolymers such as Kelzanavailable from the Kelco Company EXAMPLE NO. 1

Tests were made of an available petroleum sulfonate having an averagemolecular weight of 460/470 in designing an aqueous chemical floodingprocess for a sandstone reservoir at 95F. The tested sulfonate was oilsoluble and only very slightly soluble in water. Following the teachingin .l. Reisberg, G. Smith and .l. B. Lawson US. Pat. No. 3,508,6l2,NEODOL 25-38 was used to disperse the sulfonate in water and provide theneeded multivalent ion tolerance for field application. An apparentlysatisfactory system was developed using a blend of two waters availablein the field. This com bination field brine contained 7700 PPM totaldissolved solids. About 800 PPM of this total was calcium, barium andmagnesium ions present in the brine as the soluble chlorides withsubstantially the balance of the dissolved solids being sodium chloride.This system was tested in Berea core 776. The core had been saturatedwith water produced from the field, flooded with field crude oil andwaterflooded with produced water to residual oil saturation. Theproduced water used in these operations, one of two available in thefield contained about 77,000 PPM total dissolved solids (8,000

PPM Ca", Ba" and Mg). Oil production started after injection of aboutone-fourth pore volume of chemical slug and was maintained at asubstantially constant rate until about 0.9 pore volume of cumulativeinjection. After this time, the oil rate began to decline. Emulsionswere observed in the produced fluids at about 1.2 pore volumes ofinjection. Shortly after emulsion production was observed, the oil ratedeclined to a substantially constant but very low level (oil cuts about1 percent). All of this oil was in emulsified form and production at theconstant low rate was continuing when the test was terminated after morethan 2.] pore volumes of total injection. At termination, slightly over75 percent of the residual oil had been recovered with about 62 percentof the residual oil recovered before emulsion were observed. Analysis ofeffluent samples for sulfonate and chloride ion showed that the injectedslug was being bypassed by following fluids with only smallconcentrations of sulfonate being pulled" along with these fluids.Chloride ion data indicated that the high salinity water originally inthe core was not produced as a continuous bank. These observationsindicate that emulsions of oil, core water and chemical slug were beingbypassed by chemical flood drive solutions.

Chemical systems were designed with the oil soluble sulfonate andPYRONATE 50 mixed with NEODOL 25-38. Emulsions of reservoir crude oiland produced water with these new chemical systems were compared withemulsions of the same fluids with the oil soluble sulfonate/NEODOL 25-35system. These comparisons indicated that emulsions were still formed butthat their properties had been modified toward both reduced viscosityand more rapid coalesence. The new system was tested in Berea ore 775.Experimental conditions for this test were similar to those used for thetest of the original system in core 776 with the following minorexception: as the water soluble sulfonates served also to reduce theamount of NEODOL 25-38 required to disperse the oil soluble sulfonate,systems were adjusted to take advantage of this opportunity to decreasethe amount of this relatively expensive component of the chemical slug.

Both core 775 and 776 were 2 inch diameter by 20 inches in length. Eachwas saturated with the 77,000 PPM TDS produced water, flooded with oiland waterflooded to residual oil saturation with produced water. Thechemical flood following waterflood was conducted with the cores in thevertical position to minimize gravity effects and comprised thesequential injection of two slightly different chemical slugs, athickened'water slug and a final water drive. The two chemical slugsdiffered mainly in that the second slug had more mobility control agentand was thus less mobile than the first chemical slug. This practice hasthe effect in cores of the present length of allowing emulsions to begenerated with the higher mobility slug then driving these emulsionswith a relatively lower mobility portion of chemical slug. Thus theemulsions should be moved through the core with little bypassing unlesstheir mobility is extremely low. ln this case, the second slug willfinger through the initial emulsions and generate additional emulsionsitself.

Table No. 1 summarizes pertinent core data, chemical slug compositionand actual flooding sequence and volumes. FIGS. 1, 2 and 3 summarize theproduction data. Oil production data (FIG. 1) shows the sharp reductionin bypassed oil which occurred with the modified slug. Using theoriginal slug in core 776, 2.0-

pore volumes of total injection were required to produce about 78percent of the residual oil. This same percent of residual oil wasproduced from core 775 with the modified chemical slug after only l.2pore volumes of injection. As 0.8 pore volumes injection into many oilreservoirs would require several years,

the original oil had been produced from the two cores (67 percent for775 and 64 percent for 776) but the emulsion mobility in 775 had beensubstantially increased by the inclusion of PYRONATE 50 in the chemicalslug formulation. As a result of this increase in emulsion mobility, itwas possible for the latter part of the chemical slug and its followingthickened slug to move the oil emulsion out of the core. Thisinterpretaslug was dispersed and, toa large degree, bypassed by thefollowing fluids in this core.

EXAMPLE NO. 2

In Example No. I, it was found that inclusion of PYRONATE 50 in thechemical slug formulation resulted in increasing emulsion mobility sothe emulsion could be displaced from the core but did not substantiallyincrease the amount of residual oil recovered in the form of cleanwater-free oil. It is desirable to eliminate or avoid emulsions bothbecause of the added cost and inconvenience of treating producedemulsions to recover saleable oil and because the possibility existsthat stable emulsions persisting for long times in the reservoirenvironment might undergo inversion or other changes which would convertthem back to viscous emulsions. Tests were made of means for effectivelyreducing emulsion formation. Screening tests indicated that isobutylalcohol (lBA) was uniquely effective in increasing the coalesence rateof emulsions of typical chemical slugs, formation waters and crude oilwhich were formed in laboratory glassware by mechanical agitation. Table2 gives a typical comparison of coalesence rates for emulsions obtainedfrom chemical slugs with and without lBA (lso-butyl alcohol). In orderthat these tests be made on systems having comparable interfacialactivity, total dissolved solids in the lBA system were reduced.Chemical slug A (Table 2) was identical in TABLE 1 Core Number 776 775Core Data Porosity percent 2 l .1 20.7 Permeability millidarcies 665 580Residual Oil Saturation percent 41 38 Chloride [on in Water meg Gm. 1.261.26

Chemical Slug Composition A B A B Petroleum Sulfonate 460/470 meg/Gm.0.034 0.034 0.017 0.0 l 7 Neodol 25-318 percent Pyronate 50 meg/Gm. L00.6 0.6 0.2 Pyronate 50 meg/Gm. 0.017 0.0l7 Pusher 700 PPM 200 400 200400 Chloride lon meg/Gm. 0.l l 0.09 0.12 0.l l

Thickened Slug Composition Pusher 700 PPM 300 300 Chloride lon meq/Gm.0.001 0.00l

Final Water Drive Chloride lon meq/Gm. 0.001 0.00l

Chemical Flood Volumes Chemical Slug A pore Vol. 0.29 0.23 Chemical SlugB pore Vol. 0.22 0.25 Thickened Slug pore Vol. 0.75 0.78 Water Drivepore Vol. 0.73 0.82

composition with slug A in Example 1. Chemical slug C (Table 2) wasidentical to A except that total dissolved solids were reduced from7,700 PPM to 6,200 PPM for the reason stated above and 8 percent byweight of [8A was substituted for an equivalent amount of water.

TABLE 2 Crude Oil Chemical Fraction of Oil Free After (hrs) Vol. I, Slug0.083 0.5 l 3 24 23 A 0 0 0 0.02 0.04 C L0 L0 L0 L0 1.0 35 A 0 '0 0 0.0!0.05 C 1.37 L0 1.0 L0 1.0 50 A 0 0 0 0 0.02 C 1.4 0 .0 1.0

. 1.0 l A number in excess of 1 indicates that oil contained water oraqueous phase.

To determine if the highly effective coalesence observed in laboratoryglassware could be realized in practice, these two chemical systems weretested in 2 inch diameter by 10 inch long Berea cores (794 and 800)which were at waterflood residual oil saturation. Conditions duringthese two experiments were such that emulsification would be encouraged.These conditions are: (l) the chemical slugs were viscously unstable,(2) cores were horizontal allowing gravity effects to augment theinstability effects and encourage mixing and bypassing and (3) the floodrates were equivalent to a frontal advance rate of about 5 feet per daythus increasing shear effects above the levels which would normally beencountered in oil reservoirs, at points removed from injection orproducing wells. For the core tests, both chemical slugs were preparedin identical field brine containing about 7,700 PPM total dissolvedsolids. Forty nine percent of the oil produced from core 794 byinjecting chemical slug A (without lBA) was in the form of a stableemulsion. This result contrasts sharply with the complete absence ofstable emulsions in the production from core no 800 (slug containinglBA). Total oil production from each core was substantially equal.

EXAMPLE NO. 3

A chemical flooding slug was prepared with a composition as follows: (I)0.036 meq/gm. of a 460/470 molecular weight sodium petroleum sulfonate(2) l percent by weight of NEODOL 25-38, (3) 1350 PPM of the biopolymerKELZAN M (sold by Kelco Co.), and (4) a blend of two field waters togive a solution containing about 3,800 PPM of total dissolvedelectrolyte. Emulsions of this chemical slug and a field crude oil wereprepared in laboratory glassware and examined. The emulsions separatedinto an oil phase, an emulsion phase and an aqueous phase within about 2hours but the emulsion phase persisted throughout the 12-hourobservation period. Another chemical slug, differing only in that thissecond slug contained 4 percent by weight of iso-butyl alcoholsubstituted for an equivalent amount of water was prepared. Emulsions offield crude with this second chemical slug broke in less that 2 hoursinto a clean oil phase and a clear phase containing most of thesurfactant lBA, and a small fraction of solubilized oil.

The emulsion samples from both chemical slugs were mechanically mixed,transferred to a Brookfield viscometer and their viscosities determinedat F. and at a shear rate of 7.3 Sccl FIG. 4 gives these data as theratio of emulsion viscosity to the parent chemical slug viscosity as afunction of oil fraction in the emulsion. It is apparent that the useof4 percent lRA modities the emulsion stability and viscosity. Bothmodifications are favorable for improved oil recovery from use of thelBA-containing slug in a chemical flooding process.

EXAMPLE N0. 4

A chemical slug was prepared for application in a chemical flood processin a sandstone reservoir at l95F. The formulation of this slug was asfollows: (I) 0.02 meq/gm. or 1.65 percent of a sodium petroleumsulfonate having an average molecular weight of 540, (2) 1 percent byweight of a highly polyethoxylated long chain alcohol, NEODOL 25-30,available from Shell Chemical Company, (3) a blend of two availablefield waters blended to obtain a water containing about l3,000 PPM oftotal dissolved solids and comprised substantially of sodium chloridebut containing a total calcium, barium, and magnesium ion content ofapproximately 560 PPM.

Emulsions of this system with the crude oil from the reservoir wereexamined at 195F. and found to be 6 to 8 times as viscous as the parentchemical slug.

A second chemical slug, identical with the first except that 0.4 percentby weight of ETHODUOMEEN T/25, an ethoxylated diamine available fromArmour Chemical Company was added as an emulsion modifier. Emulsions offield crude with this second chemical slug were found to be only 0.8 to1.4 times the viscosity of the parent chemical slug.

Two Berea cores (2 inch diameter by 20 inches length) havingapproximately 2L5 percent porosity and 650 millidarcies permeabilitywere selected, saturated with the same brine used to prepare thechemical slugs, and brought to a temperature of 195F. in an air bath.The cores were flooded with field crude oil, and waterflooded toresidual oil saturation with the aforementioned brine. Residual oilsaturation was 38 percent in core 820 and 39 percent in core 830. Core820 was used to test the performance of the aforementioned slug whichdid not contain ETHODUOMEEN T725 and core 830 was used to test the slugcontaining ETHODUOMEEN T/25. In each case, a fractional pore volume slugof the chemical system was injected into the bottom of the verticallypositioned core. Chemical slugs were followed by a thickened watersolution of 1,500 PPM KELZAN M in the blended brine used to prepare thechemical slugs. Pressure differential across the core was recordedcontinuously during the experiment and produced fluids were observed forevidence of produced emulsion and were analyzed for total oil contentincluding any oil produced in emulsions. Pressure differential and oilcut are given in FIG. 5. Produced samples from core 820 did not show anyevidence of emulsion or of surfactant during the production periodcovered by the figure. lnjection continued in this core for anadditional 1.1 pore volumes during which time oil cuts again rose to ashigh as percent with most of the later oil being in the form of anemulsion with surfactant. Fifty six percent of the original residual oilwas finally recovered from the core. it is apparent from the pressuredata given in FIG. 4 and the observations recorded above that thechemical slug formed emulsions in core 820 immediately upon entering thecore. This emulsion was much more viscous than the slug so the slugfingered through or bypassed the emulsion and displaced some oil to theoutflow end of the core. Ultimately all of the slug (0.48 pore volumes)was dispersed in the core as bypassed emulsion.

The thickened slug used in both cores had a viscosity at the reservoirtemperature of 195F. which was 6 times the viscosity of the chemicalslug for 820. Since this high viscosity drive did not effect a pistonlike displacement of the emulsion. it must be concluded that theemulsion had an even higher viscosity.

Contrasting sharply with the performance of the chemical flood in core820, the chemical slug containing the emulsion modifier (ETHODUOMEENT/25) gave significantly improved performance in core 830. Oilproduction started significantly earlier, reached higher produced cutsand was sustained for a longer period of time. Total oil produced from830 during 1.67 pore volumes of injection was 62.5 percent of theoriginal residual oil in the core. This contracts with only 23 percentof residual produced from 820 after the same period of injection.Ultimate production was 56 percent of residual for the slug withoutemulsion modifier but 2.65 pore volumes of injection were required. Thedistinct advantage of the emulsion modifier is apparent.

EXAMPLE NO. 5

A chemical slug using sodium petroleum sulfonate of average molecularweight 460/470 was developed for application in a F. sandstone reservoircontaining water with about 3,900 PPM total dissolved solids. Emulsionsof the crude oil in chemical slugs prepared from the specified sulfonateand NEODOL 25-38 were viscous and stable. PYRONATE 30, a water solublesodium sulfonate (Witco Chemical Co.) was incorporated in the slugs withdistinct reduction in the emulsion viscosities and some improvement inemulsion coalesence rates. Examination of the effect of includingtertiary butyl alcohol, iso propyl alcohol and ethyl alcohol in the slugformulation showed that in general emulsion viscosities were reduced butcoalesence rates were not substantially affected. lso butyl alcoholresulted in both reduced viscosity and more rapid coalesence.Considering both economics and technical performance, the optimum slugfor this particular reservoir and its associated crude oil and water wasdetermined to be of the following composition:

Active ingredient Component Percent Wt% Meq/Gm. Sodium PetroleumSulfonate 460/470 62.5 2.1 0.03 Pyronate 30 330/350 30 3.1- 0.02 Neodol25-3S 440 59 0.8 0.01 [so butyl alcohol l00 0.6

in addition, the slug had 800 PPM of PUSHER 700 for mobility control andI60 PPM of DOWlClDE G to prevent biological attack on any of the slugcomponents. The slug was designed to be both viscously stable andchromatographically balanced.

A slug having the above formulation and comprising a volume equal to0.276 pore volumes of the 2 inch diameter by 30 inch length Berea corenumber 897 was injected into the horizontal core which had previouslybeen saturated, flooded with crude oil and waterflooded to a residualoil saturation of 36 percent. The chemical slug was followed by athickened slug containing 500 PPM PUSHER 700 in an available fresh lakewater. Oil production started after 0.13 pore volumes of injection.Traces'of chemical slug was produced at 0.85 pore volumes of totalinjection at which time 78 percent of the residual oil had beenproduced. Production of chemical slug, oil and emulsions of thesecomponents were produced until 96 percent of the residual oil had beenproduced at 1.36'pore volumes of cumulative injection. No further oilwas produced (final residual oil saturation of one percent), but someadditional chemical was produced. The oillernulsion/chemical slugproduction fluids between 0.78 Vp and 1.36 Vp injection were heated to195F, examined and found to be clean oil and an aqueous phase only.After cooling to room temperature, the oil and aqueous phases wereseparated.

What is claimed is:

1. In a process for displacing an oil within a subterranean reservoir,the improvement comprising:

testing oil displacement properties of an active aqueous surfactantsystem relative to said reservoir at the reservoir temperature; wheresaid tests indicate an oil displacement efficiency reduction due to aproduction of a low mobility emulsion, adding an emulsion modifier tosaid surfactant system formulation and applying said tests to theso-modified surfactant system; and

displacing said oil within said subterranean reservoir by injecting intothe reservoir an active aqueous surfactant system containing an emulsionmodifier that avoids the deleterious effects of an emulsion formed insaid reservoir by a dilute active aqueous surfactant system.

2. The process of claim 1 in which said emulsion modifier is apreferentially water soluble semipolar organic compound.

3. The process of claim 2 in which said emulsion modifier is isobutylalcohol.

4. The process of claim 3 in which said surfactant system contains amixture of petroleum sulfonates or the alkali metal or ammonium salts ofpetroleum sulfonates and sulfated polyalkoxylated alcohol surfactants orpoly-alkoxylated alcohol surfactants.

5. The process of claim 4 in which said surfactant system contains awater soluble polyelectrolyte thickener.

6. The process of claim 5 in which said thickener is a partiallyhydrolyzed polyacrylamide.

7. The process of claim 5 in which said thickener is a biopolymer.

8. The process of claim 1 in which said emulsion modifier is apreferentially water soluble anionic surfactant.

9. The process of claim 1 in which said emulsion modifier is a mixtureof preferentially water soluble sulfonates or the alkali metal orammonium salts of preferentially water soluble sulfonates.

10. The process of claim 9 in which said surfactant system contains amixture of petroleum sulfonates or the alkali metal or ammonium salts ofpetroleum sulfonates and sulfated polyalkoxylated alcohol surfactants orpolyalkoxylated alcohol surfactants.

11. The process of claim 10 in which said surfactant system contains awater soluble polyelectrolyte thickener.

12. The process of claim'll in which said thickener is a partiallyhydrolyzed polyacrylamide.

13. The process of claim 11 in which said thickener is a biopolymer.

14. The process of claim 1 in which said emulsion modifier is a cationicsurfactant.

15. The process of claim 14 in which said emulsion modifier is apolyethoxylated diamine.

16. The process of claim 15 in which said surfactant system contains amixture of petroleum sulfonates or the alkali metal or ammonium salts ofpetroleum sulfonates and sulfated polyalkoxylated alcohol surfactants orpolyalkoxylated alcohol surfactants.

17. The process of claim 16 in which said surfactant system contains awater soluble polyelectrolyte thickener.

18. The process of claim 17in which said thickener is a partiallyhydrolyzed polyacrylamide.

19. The process of claim 17 in which said thickener is a biopolymer.

20. The process of claim 1 in which the emulsion modifier is asynergistic mixture of preferentially water soluble semi-polar compoundsand surfactants.

21. The process of claim 20 in which said emulsion modifier is asynergistic mixture of isobutyl alcohol and water soluble sulfonates oralkali metal or ammonium salts of water soluble sulfonates.

22. The process of claim 21 in which said surfactant system contains amixture of petroleum sulfonates or the alkali metal or ammonium salts ofpetroleum sulfonates and sulfated polyalkoxylated alcohol surfactants orpolyalkoxylated alcohol surfactants.

23. The process of claim 22 in which said surfactant system contains awater soluble polyelectrolyte thickener.

24. The process of claim 23 in which said thickener is a partiallyhydrolyzed polyacrylamide. I

25. The process of claim 23 in which said thickener is a biopolymer.

2. The process of claim 1 in which said emulsion modifier is apreferentially water soluble semipolar organic compound.
 3. The processof claim 2 in which said emulsion modifier is isobutyl alcohol.
 4. Theprocess of claim 3 in which said surfactant system contains a mixture ofpetroleum sulfonates or the alkali metal or ammonium salts of petroleumsulfonates and sulfated polyalkoxylated alcohol surfactants orpoly-alkoxylated alcohol surfactants.
 5. The process of claim 4 in whichsaid surfactant system contains a water soluble polyelectrolytethickener.
 6. The process of claim 5 in which said thickener is apartially hydrolyzed polyacrylamide.
 7. The process of claim 5 in whichsaid thickener is a biopolymer.
 8. The process of claim 1 in which saidemulsion modifier is a preferentially water soluble anionic surfactant.9. The process of claim 1 in which said emulsion modifier is a mixtureof preferentially water soluble sulfonates or the alkali metal orammonium salts of preferentially water soluble sulfonates.
 10. Theprocess of claim 9 in which said surfactant system contains a mixture ofpetroleum sulfonates or the alkali metal or ammonium salts of petroleumsulfonates and sulfated polyalkoxylated alcohol surfactants orpolyalkoxylated alcohol surfactants.
 11. The process of claim 10 inwhich said surfactant system contains a water soluble polyelectrolytethickener.
 12. The process of claim 11 in which said thickener is apartially hydrolyzed polyacrylamide.
 13. The process of claim 11 inwhich said thickener is a biopolymer.
 14. The process of claim 1 inwhich said emulsion modifier is a cationic surfactant.
 15. The processof claim 14 in which said emulsion modifier is a polyethoxylateddiamine.
 16. The process of claim 15 in which said surfactant systemcontains a mixture of petroleum sulfonates or the alkali metal orammonium salts of petroleum sulfonates and sulfated polyalkoxylatedalcohol surfactants or polyalkoxylated alcohol surfactants.
 17. Theprocess of claim 16 in which said surfactant system contains a watersoluble polyelectrolyte thickener.
 18. The process of claim 17 in whichsaid thickener is a partially hydrolyzed polyacrylamide.
 19. The processof claim 17 in which said thickener is a biopolymer.
 20. The process ofclaim 1 in which the emulsion modifier is a synergistic mixture ofpreferentially water soluble semi-polar compounds and sUrfactants. 21.The process of claim 20 in which said emulsion modifier is a synergisticmixture of isobutyl alcohol and water soluble sulfonates or alkali metalor ammonium salts of water soluble sulfonates.
 22. The process of claim21 in which said surfactant system contains a mixture of petroleumsulfonates or the alkali metal or ammonium salts of petroleum sulfonatesand sulfated polyalkoxylated alcohol surfactants or polyalkoxylatedalcohol surfactants.
 23. The process of claim 22 in which saidsurfactant system contains a water soluble polyelectrolyte thickener.24. The process of claim 23 in which said thickener is a partiallyhydrolyzed polyacrylamide.
 25. The process of claim 23 in which saidthickener is a biopolymer.